A Laboratory for Early Stage Star Formation

The Perseus molecular cloud is one of the most active star-forming clouds in the solar neighborhood, spanning about 6 degrees near the southern boundary of the heroic constellation Perseus. It consists of a chain of dusty dark regions and luminous star-forming regions. Two active star-forming regions are eye-catching: IC348 (center) and NGC1333 (upper right). Credit: APOD (http://apod.nasa.gov/apod/ap111021.html)

Molecular gas fraction image derived by Lee, Stanimirovic, and their collaborators. The spatial variation of molecular gas fraction is seen on sub-pc scales (~0.4 pc).

Stars in the Milky Way and all other galaxies are made up of the interstellar medium, the gas and dust that fill the vast space between stars. But not every flavor of interstellar gas and dust is a good fuel for star formation – only in cold, dense “molecular clouds” can the gas pressure be reduced enough to allow gravitational collapse and star birth. Understanding how these dense molecular clouds form out of diffuse atomic gas is an essential step toward understanding the process of star formation. Many challenges remain, largely because the major constituent of cold molecular gas--molecular hydrogen--is not easily observed.

In their recent ApJ paper, graduate student Min-Young Lee and Professor Snezana Stanimirovic, together with an international team of collaborators, investigated for the first time the process of conversion of diffuse atomic hydrogen into molecular hydrogen on sub-parsec scales by comparing multiwavelength observational data of the Perseus molecular cloud with recent theoretical predictions.

Located 300 pc from the Earth, the Perseus molecular cloud is one of the active star-forming clouds in the solar neighborhood. To estimate the molecular hydrogen abundance across Perseus, Lee and Stanimirovic used dust infrared emission. As dust grains are believed to be well mixed with gas, dust infrared emission can be used as an excellent tracer of the total hydrogen content. High sensitivity and angular resolution data from the Improved Reprocessing of the IRAS Survey (IRIS) and the Galactic Arecibo L-band Feed Array HI (GALFA-HI) Survey were used to spatially probe the molecular gas fraction (ratio of molecular to atomic hydrogen) from the center to the very outskirts of Perseus with unprecedented accuracy.

The study found that Perseus has a relatively uniform distribution of atomic hydrogen (~10 solar mass/pc²). "This is consistent with the recent theory of molecular hydrogen formation, which suggests that a minimum amount of atomic hydrogen is a key ingredient required for shielding molecular hydrogen against dissociating radiation from nearby stars," says Lee.

The study also revealed a remarkably strong correlation between the molecular gas fraction and the total hydrogen content, supporting a theoretical scenario focusing on the equilibrium between the formation and dissociation of molecular hydrogen. This result is surprising, considering that molecular clouds are complex environments where many physical processes work together: gravitational contraction, gas accretion, outflows from newborn stars, supernovae explosion, and turbulence. Recent numerical studies that emphasize the role of turbulence in the formation of molecular hydrogen predict a very different time evolution of the molecular fraction from the equilibrium-based theory. "The Perseus investigation suggests that turbulence, which is ubiquitous in the interstellar medium and is believed to dominate the dynamics in molecular clouds, is surprisingly not very important for the formation of molecular hydrogen," Stanimirovic explains.